Added submodule for checking out openpgp-api-lib

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "openpgp-api-lib"]
+    path = libraries/openpgp-api-lib
+    url = https://github.com/open-keychain/openpgp-api-lib.git

README.md

@@ -18,6 +18,12 @@ Libraries
 This project uses three libraries:
 
 - [OpenKeyChain](https://github.com/open-keychain/open-keychain) for encryption and decryption of passwords
+    To download the library, run the following commands at the root of the project
+
+        git submodule init
+        git submodule update
+
+
 - [JGit](http://www.eclipse.org/jgit/) a pretty good git lib 
 - [Apache's FileUtils](https://commons.apache.org/proper/commons-io/) for files manipulations